The present invention generally pertains to injection molding of thin-wall plastic products and is particularly directed to preventing the formation of void-based irregularities in thin-wall portions of such products.
Some thin-wall portions of some plastic products are injection molded by using a mold cavity that includes flow chambers for directing injected fluid plastic material into thin-wall cavity sections located between the flow chambers to thereby form the thin-wall portions of the product.
For some products, it is desired to increase the thickness of the thin-cavity section in the general direction of flow within the flow chambers adjoining said zone. However, when such thickness is so increased, void-based irregularities sometimes occur in a thin-wall portion of the product that is formed by fluid plastic material directed into a zone of the thin-wall cavity section that is located between opposed flow chambers adjoining such zone because the injected fluid plastic material that is directed into such zone may surround one or more gaseous voids within such zone during the formation of the product, as shown in FIG. 1.
FIG. 1 illustrates a sequential flow-front pattern of directed plastic material within a region of a thin-wall cavity section 10 adjacent the downstream extremities of a pair of opposed flow chambers 12. The sequential flow-front pattern was determined by injecting different quantities of fluid plastic material into the flow chambers 12 and recording the extent of the resultant flow of such material directed into the thin-wall cavity section for each injection. It is seen that within a zone of the thin-wall cavity section 10 that is located between the flow chambers 12 the injected fluid plastic material that is directed into the thin-wall cavity section 10 surrounds a gaseous void 14 during the formation of the product. Consequently the thin-wall of the product includes an irregularity in the portion thereof that is formed in the zone of the thin-wall cavity section 12 in which the void 14 was surrounded by the directed fluid plastic material during the formation of the product. Such an irregularity may be manifested as a hole and/or a fragile and/or miscolored wall in such portion of the product. When the product is a container, such as a drink cup, a hole in the wall will enable fluid to leak from the container.
We have discovered that the formation of such void-based irregularities in the thin-wall portion of the product can be prevented within the zone of the thin-wall cavity section that is located between the flow chambers in which the thickness of the thin-wall cavity section increases in the general direction of flow within the flow chambers adjoining said zone by dimensioning the mold cavity so that said thickness increases in said direction at less than a threshold rate. When said thickness increases in said direction at a rate that equals or exceeds the threshold rate, such void-based irregularities usually occur. The threshold rate for a zone is determined empirically for each product, as described below in relation to the detailed description of the preferred embodiments.
The present invention provides a mold for injection-molding a product that includes at least one thin wall, comprising: a plurality of mold parts, which when combined define a mold cavity for forming the product and at least one gate from which fluid plastic material may be injected into the mold cavity; wherein the mold cavity includes at least one thin-wall cavity section and at least two opposed flow chambers that adjoin opposite edges of the thin-wall cavity section for directing injected fluid plastic material from exit positions of the said at least two opposed flow chambers into corresponding entrance positions of the thin-wall cavity section to thereby form at least one thin-wall portion of the product; wherein the at least one thin-wall cavity section includes at least one zone that is located between said at least two opposed flow a chambers; wherein within the at least one zone inscribed-sphere dimensions at each entrance position are smaller than inscribed-sphere dimensions at the corresponding exit position of the adjoining flow chamber; and wherein within the at least one zone the thickness of the at least one thin-wall cavity section increases in the general direction of flow within the flow chambers adjoining said zone, with said increase being at less than a threshold rate to thereby prevent injected fluid plastic material so directed into the at least one zone from at any time surrounding any gaseous void within the at least one zone. xe2x80x9cInscribed-sphere dimensionsxe2x80x9d are those dimensions in which a sphere can be fitted.
The present invention also provides a method of injection-molding a product that includes at least one thin wall, comprising the steps of:
(a) combining a plurality of mold parts to define a mold cavity for forming the product and at least one gate from which fluid plastic material may be injected into the mold cavity, wherein the mold cavity includes at least one thin-wall cavity section and at least two opposed flow chambers that adjoin opposite edges of the thin-wall cavity section for directing injected fluid plastic material from exit positions of the said at least two opposed flow chambers into corresponding entrance positions of the at least one thin-wall cavity section to thereby form at least one thin-wall portion of the product, wherein the at least one thin-wall cavity section includes at least one zone that is located between said at least two opposed flow chambers, and wherein within the at least one zone inscribed-sphere dimensions at each entrance position are smaller than inscribed-sphere dimensions at the corresponding exit position of the adjoining flow chamber; and
(b) injecting fluid plastic material from the gate into the mold cavity to form the product;
wherein step (a) comprises combining mold parts that define a said mold cavity in which within said at least one zone of the at least one thin-wall cavity section the thickness of the at least one thin-wall cavity section increases in the general direction of flow within the flow chambers adjoining said zone, with said increase being at less than a threshold rate to thereby prevent injected fluid plastic material so directed into the at least one zone from at any time surrounding any gaseous void within the at least one zone.
Additional features of the present invention are described with reference to the detailed description of the preferred embodiments.